The present invention relates to a process for the efficient removal of phosphorus from industrial waste water streams. The process of this invention is particularly effective for treating waste waters at low temperatures, i.e. at waste water temperatures of from about 0.degree. C. to about 10.degree. C. This process thus provides an effective method for reducing the phosphorus contamination in waste ponds and streams during periods of cold weather.
Elemental phosphorus is produced commercially in an electric arc furnace by the thermal reduction of phosphate ore in the presence of coke and silica. The gaseous mixture generated by this reaction comprises phosphorus, water vapor, fluorosilicates, and other gaseous impurities, as well as entrained fine solid particles. Large quantities of water are consumed to cool the gases from the furnace and condense the phosphorus. Additional quantities of water are used during the process to protect the phosphorus from contact with the atmosphere and possible rapid oxidation and combustion.
The waste water effluent from the producing plant is frequently stored in holding ponds adjacent to the plant. The amount of water lost from the ponds through evaporation is usually sufficient to balance the water added to the ponds as plant effluent. However, during rainy periods which frequently occur in the winter on a seasonal basis, the additional precipitation causes the water in the pond to rise to a level which requires its discharge into public waterways.
This waste water typically contains dissolved phosphorus salts, generally in the form of ortho-phosphates, reduced and organic phosphates, suspended or colloidal phosphorus, and other inorganic solid materials that might be carried over from the furnace. The total phosphorus content of the waste water must be reduced prior to its safe discharge into the environment.
Sources of phosphorus-containing waste water, in addition to the waste water generated during phosphorus production, include water used in other operations that utilize phosphorus, such as water used in the loading and unloading of railroad cars used to transport phosphorus, water used to convert phosphorus to other chemical entities including phosphorus pentoxide, phosphoric acid, phosphorus pentasulfide, etc., or any other type of industrial waste water which contains phosphorus, regardless of its origin. The term "industrial waste water", as used herein, is therefore intended to include effluent waters generated in any of these industrial operations.
Recent concerns over the amount of phosphorus discharged into the environment have been prompted not only by phosphorus employed in manufacturing operations but in the commercial and private use of large quantities of phosphate compounds, such as detergent builders used in cleaning and washing. This concern has stimulated efforts to limit the discharge of phosphorus and phosphates into streams and lakes. Consequently, environmental regulations for industrial generators of phosphorus and phosphorus compounds have become increasingly more stringent concerning the amount of phosphorus which is permitted in plant discharges.
Several techniques are currently used to remove phosphorus and phosphates from industrial waste waters. See, for example, commonly assigned U.S. Pat. No. 4,402,833, issued Sept. 6, 1983, the disclosure which is incorporated by reference herein. This patent discloses the use of lime to precipitate soluble ortho-phosphates from industrial waste water effluent. The use of aeration immediately after the addition of lime to assist in precipitation is also disclosed, as is the use of activated carbon as an adsorbant in the final stage of treatment. The use of cationic polyelectrolytes and chlorination are also effective in reducing the level of phosphates. See U.S. Pat. No. 4,043,910 and British Pat. No. 888,085.
The techniques currently in use generally involve the addition of a precipitating agent to the waste water to convert soluble ortho-phosphates into insoluble phosphorus salts. The pH of the water is controlled to optimize the precipitation of insolubles, and consequent removal of phosphates.
Although these techniques have proven to be generally effective at reducing the level of phosphorus and phosphates in waste waters, newer and stricter regulations require further refinements to meet proposed environmental standards.
It is therefore an object of this invention to provide an improved process for removing phosphorus from industrial waste water which will meet existing and proposed environmental requirements under a variety of treatment conditions.